The present invention relates to a photographic camera that includes an exposure control system having scene light detection apparatus in combination with an electronically controlled source of artificial illumination, in general, and to means for varying the size of the acceptance angle of the photodetector forming a part of said apparatus wherein scene light is measured both before and during an exposure interval, in particular.
Built-in light detecting devices which form an integral part of an automatic camera exposure control arrangement are well-known to those skilled in the photographic arts. Such light detecting devices generally consist of a photovoltaic or photoresistive-type photodetector associated with an optical system so that light from a preselected area of a scene to be photographed can be directed onto the photosensitive surface of the photodetector. An electrical output signal indicative of the intensity or the brightness of the various objects located in the preselected area of the scene is derived from the photodetector and is thereafter generally utilized to control the exposure delivered to the film in accordance with sensitometric characteristics of the film and the known performance characteristics of other elements which comprise the exposure control arrangement. The electrical output signal of such devices are normally used prior to actual exposure to, for example, automatically adjust the exposure settings of the camera or after an exposure cycle is initiated to terminate the cycle upon satisfaction of a predetermined exposure condition.
The preselected area of the scene depends upon the acceptance angle of the light detecting device, the acceptance angle meaning the solid angle of the cone of light received by the light detecting device. Thus, if light from a particular scene object is to have an influence on the magnitude of the output signal of the light detecting device, that object must be located within the device's acceptance angle, i.e., the object must be "seen" by the device. It is apparent, therefore that the acceptance angle of such light detecting devices can be used as a means for controlling what the exposure will be in the sense that it can be aimed at a certain area of the scene that is considered important in preference to another area of the scene which may not be considered quite as important.
The acceptance angle employed for a particular light detecting device depends on the kinds of scenes it will probably be used to measure and the type of lighting which is used to illuminate those scenes. Consider, for example, a horizontal landscape illuminated by sky light or sun light. With this type of scene and lighting conditions, it is generally desirable to have a light detecting device which looks generally downwardly to exclude the sky whose influence, if considered, would tend to cause the ground details to be underexposed because the light detecting device would incorrectly assume that the ground detail was being illuminated to the same light level as the light level being provided by sky light.
Photographic cameras having exposure control systems that include a scene light detecting device that can be selectively aimed in different angular directions to compensate for extremes in lighting conditions within a particular scene are presently available in the prior art. In, for example, U.S. Pat. No. 4,173,400 to F. Faramarzpour there is disclosed an automatic camera having both ambient and flash exposure control modes of operation that is provided with a scene light detecting device having a particular acceptance angle that, depending upon the camera exposure mode, is aimed in different angular directions. Aiming of the light detecting device is accomplished by an arrangement that is responsive to the insertion of an artificial light source into the camera body that causes the light sensitive device to be aimed at a point above the camera's horizontal axis. However, if a scene to be photographed should include a relatively intense, but extremely localized light source, such as lighted candles on a birthday cake, pre-aiming the light sensitive device for the purpose of precluding the light sensitive device from classifying a low light level scene as one of high light level, when one cannot accurately predict where such a light source might be located, would not be practicable. If an otherwise low light level scene were to be classified as one of high light level, underexposed film would be the result.
Misclassifying a low light level scene as a high light level scene in a proportional fill-flash type of exposure control system such as that described in U.S. Pat. No. 4,192,587 to A. LaRocque et al would be particularly harmful in that it would prevent such an exposure control system from producing a properly exposed film. In a proportional fill-flash type of exposure control system, a mixture of both ambient and artificial light is employed for film exposure. The exposure control system operates in a range of light intensities such that film exposure is due to a mixture of available natural light and artificial or strobe light whose proportion changes in accordance with the intensity of the natural available light. Obviously, if the light sensing device indicates more ambient light in the scene than is actually present, such as in the birthday candle example mentioned above, less artificial strobe light will be provided and therefore an underexposed film will result.
It is a primary object of the present invention, therefore, to provide scene light sensing apparatus for an exposure control system that will minimize the possibility of a relatively intense localized light source within a scene to be photographed causing an underexposed film.
Another object of the present invention is to provide apparatus for varying the acceptance angle of a photosensitive device for use in a photographic camera where the acceptance is changed in response to the actuation of a selected camera function.
A further object of the present invention is to provide apparatus for varying the acceptance angle of a photosensitive device for use in a photographic camera that will have no effect on scene light measurement during an exposure interval.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings.